This is literally true, and covers what was important in context, but warrants a little extra explanation. Since these devices are specifically for humans to interface with (they typically have a button or contact sensor, though some have keypads or a fingerprint reader) they are logically Human Interface Device class USB devices, but they do not speak the HID Keyboard or Pointing Device sub-protocols like your mouse or keyboard (or the built-in "take a photo" button on your web cam). Instead they provide a FIDO-specific HID sub-protocol, which is publicly documented, instead of operations like "Caps Lock pressed" it's got stuff like "Begin enrolment" or "PIN xxxx entered by the user" which only makes sense for this specific problem.
They do, and an earlier version of my post mentioned that but I edited it down.
However, FIDO mode does not speak the keyboard sub-protocol. This means on the one hand it's not useable out of the box with some random device that allows USB keyboard input like the custom Yubico OTP mode is, but on the other hand it's able to deliver a good UX while having excellent security properties that would not be practical using keyboard emulation.
This is literally true, and covers what was important in context, but warrants a little extra explanation. Since these devices are specifically for humans to interface with (they typically have a button or contact sensor, though some have keypads or a fingerprint reader) they are logically Human Interface Device class USB devices, but they do not speak the HID Keyboard or Pointing Device sub-protocols like your mouse or keyboard (or the built-in "take a photo" button on your web cam). Instead they provide a FIDO-specific HID sub-protocol, which is publicly documented, instead of operations like "Caps Lock pressed" it's got stuff like "Begin enrolment" or "PIN xxxx entered by the user" which only makes sense for this specific problem.